The friction members for disk brake pads, brake shoes, clutch plates or the like for automobiles or the like are composed of a friction material to which an iron-based back material is attached to form the monolithic structure. These friction members generate a frictional force when subject to a relative motion therebetween, e.g., with the disk rotor or clutch plate it faces while being pressed to engage the brake of the automobile or to transmit the engine's driving force to the wheels. Therefore, a friction material should satisfy various requirements, e.g., high resistance to wear, high and stable coefficient of friction, high resistance to fade to have the coefficient of friction not rapidly deteriorate at high temperatures, generating little abnormal sound, and lower counter surface (hereinafter sometimes referred to as rotor) attack.
In order to satisfy these requirements, a friction material is composed of materials of various characteristics in the major basic portions of the fibrous base, binder and filler to exhibit the required frictional functions. However, a tough material highly resistant to wear and hence difficult to be worn out tends to more often attack counter surfaces. On the other hand, a soft material that attacks the plane it faces less tends to be less resistant to fade. A material highly resistant to fade, e.g., metal, has a coefficient of friction of insufficient stability. In short, a single material cannot satisfy all of the performance requirements.
The low steel type material or the semi-metallic material comprising a base of steel fibers, binder of phenolic thermosetting resin (e.g., phenol resin) and inorganic or metallic friction/wear modifier has been attracting attention due to its much higher thermal stability and resistance to wear than those of the asbestos-based friction material. The frictional characteristics of the low steel type or the semi-metallic friction material are determined by the ratio of steel fibers serving as the base, and the friction/wear modifier incorporated in the base. It is normal practice to create the desired frictional characteristics by incorporating abrasives of a high degree of hardness (e.g., alumina, chromium oxide or quartz), inorganic filler (e.g., barium sulfate or calcium carbonate), a hard metal or the like and lubricant composed of cashew dust, graphite or soft metal (e.g., copper or brass).
However, increasing the ratio of hard material, e.g., steel fibers or abrasives, to improve braking characteristics and resistance to wear may cause various problems, (e.g., wear of the rotor surface which faces the brake pad or the like and wear of the friction material formed into a given shape,) and resultant deterioration of the brake characteristics and rotor serviceability. This forms a vicious cycle in which the scratches and irregularities formed on the rotor surface cause the so-called abrasive wear to roughen the friction material shape surface and further deteriorate the frictional characteristics.
Therefore, the friction material containing steel fibers as the fibrous base has a major disadvantage of large counter surface attack against the surface which the friction material faces, although it has an advantage of good friction characteristics at high temperature.
Moreover, the semi-metallic friction material containing a high proportion of steel fibers has problems of rusting the friction material itself and the rotor. The friction material incorporating stainless steel fibers to solve the rusting problems still has problems of large counter surface attack, although the rusting problems are reduced.
Even the low-steel friction material, reduced in steel fiber content to reduce the disadvantages involved in the semi-metallic friction material, still has the problems of attacking the plane it faces and rusting.
Therefore, various friction materials containing steel fibers have been proposed to solve the above problems. These include the semi-metallic friction materials comprising steel fibers, binder and filler, as disclosed by Japanese Patent Laid-open Publication Nos. 59-24772 and 60-1278; friction materials of sintered iron type comprising iron fibers (e.g., steel fibers), iron particles or metallic binder, and dispersed with graphite particles, as disclosed by International Patent Publication No.2001-501650; friction materials for brakes, comprising fibers incorporated with a lubricant or the like, the fibers being metallic (e.g., steel or copper), having a diameter of 20 μm or more and contained at 10 to 20% by volume on the friction material for brakes, as disclosed by Japanese Patent Laid-open Publication No.8-254237; and non-asbestos-based friction materials comprising a fibrous component other than asbestos, thermosetting resin component (e.g., phenolic resin) and filler component (e.g., graphite or barium sulfate), the fibrous component containing, at least partly, steel fibers produced by vibration cutting, as disclosed by Japanese Patent Laid-open Publication No.6-129454.
In spite of these proposals, however, only a few of the so-called semi-metallic friction materials, which contain high proportions of steel-based fibers, have good coefficients of friction at high temperatures and, at the same time, lower counter surface attack. On the other hand, the friction material containing no steel fibers has the problem of insufficient coefficient of friction, which makes it impractical. The friction material containing a high proportion of lubricant to improve resistance to wear has problems that it cannot secure a sufficient coefficient of friction or resistance to wear at low and high temperature.
Therefore, there are great demands for the non-asbestos-based friction materials which have excellent friction and rust preventive characteristics, and much lower counter surface attack.
It is an object of the present invention to provide a non-asbestos-based friction material for brakes, clutches or the like for automobiles, large-size trucks, railway cars and various industrial machines, which has excellent friction and rust preventive characteristics, and attacks the plane it faces only to a limited extent.